xen/console: Add support for early printk

xen/console: Add support for early printk

On ARM, a function (early_printk) was introduced to output message when the
serial port is not initialized.

This solution is fragile because the developper needs to know when the serial
port is initialized, to use either early_printk or printk. Moreover some
functions (mainly in common code), only use printk. This will result to a loss
of message sometimes.

Directly call early_printk in console code when the serial port is not yet
initialized. For this purpose use serial_steal_fn.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Keir Fraser <keir@xen.org>